Fix #88: Timeout handling. (#91)

fix timeout handling

* Despite what is documented in sd_bus_get_timeout(3), the timeout
  returned is actually an absolute time point of Linux's CLOCK_MONOTONIC
  clock. Hence, we first have to subtract the current time from the
  timeout in order to get a relative time that can be passed to poll.

* For async call timeouts to reliably work, we need a way to notify the
  event loop of a connection that is currently blocked waiting in poll.
  I.e. assume the event loop thread entered poll with a timeout set to
  T1. Afterwards, the main thread starts an async call C with a timeout
  T2 < T1. In order for C to be canceled after its timeout T1 has
  elapsed, we have to be able to notify the event loop so that it can
  update its poll data.

Co-authored-by: Urs Ritzmann <ursritzmann@protonmail.ch>
Co-authored-by: Lukasz Marcul <lukasz.marcul@onemeter.com>

include/sdbus-c++/Message.h

@@ -49,6 +49,7 @@ namespace sdbus {
     class MethodReply;
     namespace internal {
         class ISdBus;
+        class IConnection;
     }
 }
 
@@ -195,9 +196,13 @@ namespace sdbus {
         void dontExpectReply();
         bool doesntExpectReply() const;
 
+    protected:
+        MethodCall(void *msg, internal::ISdBus* sdbus, const internal::IConnection* connection, adopt_message_t) noexcept;
+
     private:
         MethodReply sendWithReply(uint64_t timeout = 0) const;
         MethodReply sendWithNoReply() const;
+        const internal::IConnection* connection_{};
     };
 
     class MethodReply : public Message